During drilling of a borehole into the earth formation for the production of oil or gas, it frequently occurs that chemical treatment of the rock formation is required. For example in case of large losses of drilling fluid into fractures in the formation, shutting off of such fractures is necessary to prevent such further fluid losses. Such fractures may also lead to poor cementation of wellbore casing when drilling is done in overbalance mode, or to early breakout of reservoir water in case the fractures are connected to a water layer when the well is put on production. Similar problems as described above with regard to fractures can also be encountered when highly permeable zone of the earth formation are traversed during drilling, and the present invention is equally applicable to this situation. A highly permeable zone, wherein the permeability is for example at least 10 times higher than the average permeability of the earth formation that is traversed, is for example prone to early water breakthrough. Sealing off fluid communication between the borehole and the highly permeable region can therefore be desirable.
However, contamination of treatment fluid with drilling mud in the borehole during overbalanced drilling and the difficulty to place treatment fluid in the formation on the high side of the well, has negatively affected the treatment success. Injection of treatment chemical into the surrounding formation is normally avoided when drilling in the underbalance mode since such injection can only be achieved in overbalance mode, and switching to overbalance mode would necessitate the whole fluid column in the borehole becoming overbalanced.
Thus, there is a need to provide an improved method and assembly which allows placement of treatment fluid while drilling in the overbalance mode without mixing of treatment fluid with the drilling mud, and which allows placement of treatment fluid while drilling in the unbalance mode while the borehole outside the treatment zone still remains underbalanced.